Poly-p-phenylenebenzobisthiazole, poly-p-phenylenebenzobisoxazole and poly-2,5-benzoxazole are intractable polymers by which is meant they are non-melting but soluble. Mixtures of these polymers have been made with thermoplastic polymers to provide melt-processability. Mixtures of these polymers with other intractable polymers have been made for various purposes. Procedures for making these mixtures have been deficient since the available techniques for preparing solutions of the polymer combinations for processing have required isolation of the intractable polymer from its polymerization solvent, polyphosphoric acid, prior to preparation of the mixed polymer solution. The present invention overcomes these deficiencies.